Beater wheel mills are typically used for the comminution of solid fuels, such as brown coal, into particles having a grain size that is suitable for combustion, for example in the furnace of a power plant.
For example, one such type of pulverizer mill is a coal dust fan mill which grinds and conditions coal into carbon dust for the subsequent conveyance of the carbon dust into the coal dust firing of steam generators. The comminution of raw coal in such a coal dust fan mill is accomplished by a rapidly rotating beater wheel that crushes the coal fed thereto. The coal is fed into a circumferential rim region of the beater wheel and is comminuted by beater plates coupled to the beater wheel. The beater wheel typically consists of a beater wheel hub disc and a beater wheel ring disc as well as webs, or ribs, which are inserted therebetween along the radially outer part of the beater wheel hub disc and the beater wheel ring disc and bolted thereto. The beater plates are arranged on the ribs, and may be arranged as a single or a multiple part beater plate set corresponding to each rib.
As a consequence of low-NOx combustion of lignite in modern power plants, the air distribution between mill air and burner air has been shifted to increase burner air (otherwise known as secondary air), and consequently to decrease mill air (otherwise known as primary air), thus leading to lower emission values and a better environmental performance. However, the reduction of primary air in the mill decreases cooling within the mill and consequently increases the temperature loading of the mill. The operating temperatures in typical beater wheel mills can exceed 300 deg. C, and the temperatures at the bolted connections can often exceed 450 deg. C.
In conventional beater wheels, the hub disc, ring disc, webs, and bolts are fabricated using heat-resistant steel or steel castings. Due to the material properties of the steel, the bolts used in conventional beater wheel mills may tend to exhibit creep at the high operating temperatures typically reached within the mill. Additionally, in combination with a relatively low pretension of the bolted connection, due to weakness of material pairs, an undesired loss of pretension after a certain operation period may also occur. Moreover, in the high temperature environment of a mill, gaps may gradually form in the bolted connections. Such gaps may, in some cases, be large enough to collect coal dust therein, promoting further plastic deformation of the structure.
What is needed, therefore, is an improved beater mill wheel and method for assembling a beater wheel for a pulverizer mill.